Chess changes: en passant and not suggesting illiegal moves

[Tyson Williams]

Hello Johnny,

Thanks for the links.  I will take a look at them.

To help you and others better understand the cases, it just occurred to me
that I could create and markup some screenshots.  I should be able to get
those over the weekend.


On Fri, Jun 5, 2020, 2:48 PM Johnny Jazeix <jazeix at gmail.com> wrote:

> Hi Tyson,
>
> thank you for your explanation. We are using the engine of p4wn
> (https://github.com/douglasbagnall/p4wn) to power the chess activity.
> There were already a few bug they fixed for "en passant":
> https://github.com/douglasbagnall/p4wn/issues?q=is%3Aissue+is%3Aclosed
> but maybe that did not handle all the cases.
>
> If you have knowledge, for sure these improvements are welcome!
> If you can reproduce the bugs in http://p4wn.sourceforge.net/, then it
> would be better if this can be fixed upstream (proposing a pull
> request) and we'll have to report on our code
> (
> https://github.com/gcompris/GCompris-qt/blob/master/src/activities/chess/engine.js
> ).
> I can help you understand the code on GCompris, but will have more
> difficulty to help on the engine.
>
> I'll take a closer look tomorrow to try to understand more the
> different use cases you are talking about.
>
> Johnny
>
> Le ven. 5 juin 2020 à 14:18, Tyson Williams <tyson.d.w at gmail.com> a écrit
> :
> >
> > !Hello everyone,
> >
> > My children (twin 6yos and 4yo) love playing the activities in
> GCompris.  In particular, one of my 6yos especially enjoys that he can been
> me at the flying kings variant of checkers.
> >
> > Anyway, there are two improvements that I would like to make to the
> chess activities.
> >
> > The first is that en passant is not a supported move.  I would like to
> add support for them.
> >
> > The second is about the inconsistency in which moves are displayed as
> possible.  After clicking on a piece, the general behavior is that the
> squares to which that piece can move are highlighted.  The suggested moves
> vary according to the situation in many cases.
> >
> > Here are five cases in which only legal moves are suggested:
> > - Opponent pieces in the way (e.g. if R on a1 can capture at a2, then
> capturing at a3 is not suggested)
> > - Own piece in the way (e.g. there are no suggested moves for the king
> as the first move of the game)
> > - Pawn's first vs non-first move (i.e. only when it is a pawn's first
> move and the two squares in front of it are empty is the suggestion given
> to move the pawn forward two squares)
> > - Pawn capture (i.e. only if an enemy piece is diagonally in front of a
> pawn is the suggestion given to move in that diagonal direction)
> > - Castling (i.e. castling the king by moving it horizontally is only
> suggested if it is valid...which means the king has never moved, the rook
> as never moved, square the king moves through is not being attached, and
> the square the king stops on is not being attached)
> >
> > In contrast, there is one case in which invalid moves are suggested,
> which is when the king of that player would become under attack (except for
> castling as mentioned above).  I know of four ways in which this can happen:
> > - Suggestions are given to move the king to a square that is under
> attack.
> > - Suggestions are given to move a piece that is absolutely pinned.
> > - For a piece that can put itself into an absolute pin, suggestions are
> given to move that piece that would not place it into an absolute pin.
> > - For a piece that can capture a piece attacking the king, suggests are
> given to move that piece that does not capture that piece or place it into
> an absolute pin.
> >
> > If the user clicks such an illegal suggestion, the follow message is
> displayed.
> > > "Invalid, your king may be in check"
> >
> > This message makes me think that I have missed some case....that the
> game can suggest a move that is illegal for some reason other than the king
> being under attack after the move.  However, I have been unable to find
> such a case.
> >
> > When my children play chess and their king becomes checked, they click
> the suggested moves to get out of check, but many of them are illegal and
> display that message to them.  I would prefer if those illegal moves were
> not suggested.
> >
> > Would making en passant possible and removing illegal move suggestions
> be welcome changes to the chess activities?
> >
> > --
> > Tyson Williams
>
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